docs(sdk): add comprehensive SDK documentation and wire format reference

Covers all official SDKs (Rust, Go, Python, TypeScript, C FFI),
the v2 wire format with method ID tables, authentication flow,
and a build-your-own-SDK guide with implementation checklist.

docs/sdk/go.md

@@ -0,0 +1,95 @@
+# Go SDK
+
+The Go SDK provides a native QUIC client with Cap'n Proto serialization.
+
+Location: `sdks/go/`
+
+## Installation
+
+```sh
+go get quicproquo.dev/sdk/go
+```
+
+## Quick Start
+
+```go
+package main
+
+import (
+    "context"
+    "fmt"
+    "quicproquo.dev/sdk/go/qpq"
+)
+
+func main() {
+    ctx := context.Background()
+
+    client, err := qpq.Connect(ctx, qpq.Options{
+        Addr:               "127.0.0.1:5001",
+        InsecureSkipVerify: true, // dev only
+    })
+    if err != nil {
+        panic(err)
+    }
+    defer client.Close()
+
+    // Health check
+    status, _ := client.Health(ctx)
+    fmt.Println("Server:", status)
+
+    // OPAQUE login (requires external OPAQUE library)
+    resp, _ := client.LoginStart(ctx, "alice", opaqueRequest)
+    // ... process resp with OPAQUE library ...
+    client.LoginFinish(ctx, "alice", finalization, identityKey)
+
+    // Resolve user
+    key, _ := client.ResolveUser(ctx, "bob")
+
+    // Create channel
+    chID, wasNew, _ := client.CreateChannel(ctx, key)
+
+    // Send
+    seq, _ := client.Send(ctx, key, []byte("hello"))
+
+    // Receive (long-poll)
+    msgs, _ := client.ReceiveWait(ctx, myKey, 5000)
+    for _, m := range msgs {
+        fmt.Printf("[%d] %s\n", m.Seq, m.Data)
+    }
+}
+```
+
+## API
+
+| Method | Description |
+|---|---|
+| `qpq.Connect(ctx, opts)` | Connect to server |
+| `client.Close()` | Disconnect |
+| `client.Health(ctx)` | Health check |
+| `client.SetSessionToken(token)` | Set pre-existing token |
+| `client.RegisterStart(ctx, username, request)` | OPAQUE register start |
+| `client.RegisterFinish(ctx, username, upload, key)` | OPAQUE register finish |
+| `client.LoginStart(ctx, username, request)` | OPAQUE login start |
+| `client.LoginFinish(ctx, username, finalization, key)` | OPAQUE login finish |
+| `client.ResolveUser(ctx, username)` | Look up identity key |
+| `client.CreateChannel(ctx, peerKey)` | Create DM channel |
+| `client.Send(ctx, recipientKey, payload)` | Send message |
+| `client.SendWithTTL(ctx, recipientKey, payload, ttl)` | Disappearing message |
+| `client.Receive(ctx, recipientKey)` | Fetch messages |
+| `client.ReceiveWait(ctx, recipientKey, timeoutMs)` | Long-poll |
+| `client.DeleteAccount(ctx)` | Delete account |
+
+## Structure
+
+- `qpq/` -- High-level client API
+- `transport/` -- QUIC + TLS 1.3 transport, Cap'n Proto RPC framing
+- `proto/node/` -- Generated Cap'n Proto Go types
+- `cmd/example/` -- Example usage
+
+## Transport
+
+The Go SDK uses `quic-go` for QUIC transport and `capnproto.org/go/capnp/v3` for Cap'n Proto RPC. The transport layer handles:
+
+- TLS 1.3 with configurable CA certificates
+- Cap'n Proto RPC bootstrap over QUIC streams
+- Automatic connection management